<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        var message
        alert(message);

        //下边这个变量没有声明
        // var age;
        alert(typeof  age);//会产生错误

        alert(null == undefined);

    </script>
</head>
<body>

</body>
</html>